1. Field of the Invention
The present invention relates to a motorcycle, and more particularly to a motorcycle having front end portions of a pair of right and left pipe members, each front end portion having an external shape of a prismatic cross section and which being bent sideways in an outwardly convex shape at a longitudinally intermediate position. The front end portions are connected contiguously to a head pipe, a front fork which supports a front wheel through a shaft is supported by and steerable by the head pipe, and an engine is disposed between lower portions of both the pipe members. The present invention is also concerned with the improvement of a frame structure for a motorcycle having the aforementioned structure.
2. Description of the Background Art
Applicants have determined that the background art suffers from the following disadvantages. A frame structure for a motorcycle of the background art includes a space for disposing an upper end portion of an engine to ensure adequate clearance between lower portions of a pair of right and left pipe members. Both the pipe members are formed so as to be relatively narrow in the width between their upper portions in order to make it difficult for rider's knees to come into contact with the pipe members. An example of this type of frame structure is seen in Japanese Patent Laid-open No. 2001-63667.
However, the pipe members in the aforementioned frame structure have a cross-sectional shape including lower half portions which are substantially vertical on both sides of an upper end portion of an engine and upper half portions which are bent toward a central line of a vehicle body from upper ends of the lower half portions. The pipe members are also formed by bending a stock having been subjected to extrusion. Therefore, the bent surfaces of the pipe members and their respective side walls are not orthogonal to each other and thus bending of the stock after extrusion is relatively difficult.